The present disclosure relates to gas sensors, and particularly a low-density coating for the sensing element.
The automotive industry has used exhaust gas sensors in automotive vehicles for many years to sense the composition of exhaust gases, namely, oxygen. For example, a sensor is used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio for combustion.
One type of sensor uses an ionically conductive solid electrolyte between porous electrodes. For oxygen, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, (i. e. , reference gas), is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (xe2x80x9cemfxe2x80x9d) is developed between the electrodes according to the Nernst equation.
With the Nernst principle, chemical energy is converted into electromotive force. A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (xe2x80x9cexhaust gas electrodexe2x80x9d), and a porous electrode exposed to a known gas""s partial pressure (xe2x80x9creference electrodexe2x80x9d). Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine""s exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor""s ionic conductivity at low exhaust temperatures. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (                        -          RT                          4          ⁢                      xe2x80x83                    ⁢          F                    )        ⁢          xe2x80x83        ⁢    ln    ⁢          xe2x80x83        ⁢          (                        P                      O            2                    ref                          P                      O            2                              )      
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref =oxygen partial pressure of the reference gas 
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (emf) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, conditions without quantifying the actual air-to-fuel ratio of the exhaust mixture.
In a conventional sensor, the sensor comprises a first electrode capable of sensing an exhaust gas and a second electrode capable of sensing a reference gas with an ionically conductive solid electrolyte disposed therebetween. High temperatures and materials such as silicon, lead and the like, present in engine exhaust, can poison or otherwise damage the sensing electrode. In order to prevent poisoning/damage to the sensing electrode, a protective layer made of spinel or the like, has conventionally been applied to the sensing electrode.
The protective layer is designed to allow for the electrodes to sense the particular gas without inhibiting the performance of the sensor. A thick layer (or multiple layers) of protective coating more effectively inhibits the transmission of the poisoning materials, but at the expense of a decrease in the efficiency of the sensor. Furthermore, the protective layer itself can become clogged, inhibiting passage of exhaust gases for sensing. One conventional poison resistance technique comprises applying multiple layers of a heat resistant metal oxide to the electrode to form a protective layer. However, the multiple layers have a tendency to change the performance of the sensor and only provide limited poison protection.
Accordingly, there exists a need in the art for improved protective coatings for gas sensors.
The drawbacks and disadvantages of the prior art are overcome by the low-density coating for a gas sensor and method for making the same. The method comprises mixing a metal oxide with a polymer to create a composition. The composition is applied to at least a portion of the sensing element comprising two electrodes with an electrolyte disposed therebetween, and calcined to form a protective coating. A gas sensor created according to the above-referenced method is also disclosed.